1. Field of the Invention
The present invention relates to an optical pulse timing detection apparatus, an optical pulse timing detection method, an optical pulse timing adjustment apparatus, and an optical pulse timing adjustment method. In particular, the invention relates to an optical pulse timing detection apparatus and an optical pulse timing detection method capable of detecting timing fluctuation of an optical pulse or a pulse sequence with a timing resolution on the order of femto-seconds, and an optical pulse timing adjustment apparatus and an optical pulse timing adjustment method for adjusting timing of an optical pulse or a optical pulse sequence on the basis of a result of the detection. In other words, the invention relates to a jitter skew measurement and compensation scheme in the ultra-high speed optical signal measurement technique and the ultra-high speed optical communication technique such as optical time division multiplexing.
2. Description of the Related Art
In signal processing, signal transmission and measurement using optical pulses, time fluctuation in the optical pulses becomes a chief factor which deteriorates a ratio of signal to noise and a resolution. On the other hand, it is known that electrical detection of a minute time difference is very difficult in the optical pulse measurement. For example, in the ultra-high speed OTDM (Optical Time Division Multiplexing), the pulse spacing becomes one picosecond (ps=10-12 second) or less. In the optoelectronic technique involving optoelectronic conversion, the timing fluctuation of optical pulses cannot be detected in principle. However, measurement and control of such high speed optical phenomena are techniques indispensable for implementing high speed optical pulse transmission.
At the present time, experimental verification for an ultra-high speed OTDM technique on the order of tera bits is being conducted. Frequently, in this experimental verification, a pulse light source (such as an embodiment lock laser) of 40 giga bit/sec (Gbit/sec) or 10 Gbit/sec is used. A bit sequence of 40 Gbit/s or 10 Gbit/s is generated by a combination of the pulse light source and an optical modulator. Bit sequences from N pulse light sources are provided with suitable time differences by optical waveguides, replicated and combined. As a result, the bit sequences are multiplexed at a bit rate that is N times of the original pulse light source.
In the case where verification of the fundamental performance in the dispersion compensation or the optical transmission is aimed at, it can be coped with by the above-described configuration as well. For forming time slots of the ultra-high speed OTDM by using information from separate signal sources, it is necessary to establish a technique for multiplexing signals from a plurality of signal sources while providing them with minute time differences. For example, in the case where optical pulses from signal sources 1 to 4 are multiplexed by a multiplexer MUX to obtain an OTDM signal, individual signal sources or various systems coupling them to each other are subject to disturbance caused by various causes, and consequently how optical pulses from respective signal sources are incorporated into appropriate time slots A to D of the OTDM signal properly becomes a problem. In addition, in the ultra-high speed OTDM, each time slot has only a picosecond or so, and consequently only jitter of 300 femto-second or so is allowed. In the case where jitter must be corrected thus strictly, it is necessary to measure and correct jitter in real time.
As a conventional method for detecting temporal fluctuation or discrepancy in the optical pulses, a method using nonlinear optical crystal and utilizing a sum frequency and a difference frequency proposed by F. Salin etc. is generally known (for example, F. Salin, P. Georges, G. Roger, and A. Brun, “Single-shot measurement of a 52-fs pulse,” Applied Optics, Vol. 26, No. 21, 1987, which is hereafter referred to as paper 1). According to this method, two optical pulses are incident on a nonlinear optical crystal so as to partially overlap each other in the temporal position relation, and a sum-frequency beam corresponding to the sum (or difference) of the frequencies is generated at that time. The sum-frequency beam is received by a receiver, and a point having the highest light intensity is regarded as a point at which the two optical pulses coincide with each other in temporal position. On the basis of a difference in intensity between that point and a point having discrepancy in temporal position, a time difference from the coinciding point. i.e., the temporal discrepancy between the optical pulses is calculated.
Furthermore, a timing detection circuit for ultra-high speed optical pulses including a sum-frequency optical receiver for observing the sum-frequency beam by using the method described in the paper 1 has also been proposed (see, for example, FIG. 4 and paragraphs 0063 and 0064 in Japanese Patent Application Laid-Open (JP-A) No. 2001-53684, which is hereafter referred to as paper 2). In this timing detection circuit, optical pulse timing is adjusted and controlled to be in an optimum delayed position by detecting an increase or decrease in the received signal in the sum-frequency optical receiver by means of a signal discrimination circuit and feeding back a result of the detection to an optical delay control circuit (an optical path length controller) as a delay control signal.
It is also possible to use a photo-detection element for generating photo current caused by two photon absorption, instead of nonlinear optical crystal for generating the sum-frequency beam. When utilizing photo current caused by two photon absorption, a time difference is calculated by measuring an increase or decrease in photo current caused by two photon absorption that occurs in the overlapping portion of the pulse.
However, the optical pulse timing detection method using the sum (difference) frequency or the photo current caused by two photon absorption is unsuitable for the real-time measurement. In the optical pulse timing detection method using the sum (difference) frequency or the photo current caused by two photon absorption, a peak position (pulse overlapping) is first detected and the time difference is measured by using the difference of light intensity (power in the case of two-photon absorption) on the basis of the peak position. Therefore, it is hard to grasp at a glance how large the time difference is, and the measurement costs too much labor.
Furthermore, in the optical pulse detection method using the sum (difference) frequency, fundamentally the sum (difference)-frequency beam is not coaxial with a detection subject beam. If the wavelength of the optical pulse changes, it is necessary to change the angle of the crystal and the angle of the detector, resulting in a problem of complicated adjustment. Since the nonlinear optical effect is used, there is a problem that detection is difficult in the case of an optical pulse having weak intensity.
In addition, in the optical pulse timing detection method using the photo current caused by two photon absorption, both the detection subject optical pulse and the reference optical pulse must be very strong in intensity for obtaining an on/off ratio of some degree. This results in a problem that background noise also increases.
The invention has been achieved in order to solve the above-described problems. It is an object of the invention to provide an optical pulse timing detection apparatus and an optical pulse timing detection method capable of detecting timing fluctuation in an optical pulse or an optical pulse sequence with a time resolution on the order of femto-seconds and in real time.
It is another object of the invention to provide an optical pulse timing adjustment apparatus and an optical pulse timing adjustment method capable of adjusting timing in an optical pulse or an optical pulse sequence with a time resolution on the order of femto-seconds and in real time.